Need advice, 911SC air fuel mixture setting.

[Porsche 2]

Need assistance in solving air fuel mixture setting. Have '78 SC with factory spec. rebuilt '80 engine installed for track events, with no smog control system. Engine ran well for several years at track events.

Busy installing used '80 SC factory oxygen sensor system (OXS), with all components, including harnesses, now in place. Want to put this car back on the road which requires passing smog test. Attempting to adjust air/fuel mixture approximately to drive/deliver to shop for final adjustments.

First tried to run engine with no change to previous settings. Engine started immediately, but slowly lost rpm's and stopped, within 30 seconds. First air fuel adjustment was anti-clockwise (leaner) 45 degree turn of the air/fuel mixture screw. Engine started but stopped almost immediately. Then turned the adjustment several successive clockwise 45 degree turns, restarting the engine each time, with improving engine running condition. Also adjusted idle to reduce rpm's from 1500 to 2000 to 1,000 approx. Tried more such adjustments until engine was idling smoothly, but would not start again unless I lifted the air flow "arm" in the air intake very slightly, and then it would run well without this hand assistance for up to 2 minutes, but would then stop. I have probably turned the air/flow mixture screw some 360 degrees clockwise now, with no further improvements.

Ambient air temperature is about 70 degrees F. Car has been sitting for two years with occasional warmups in the garage every few months or so. Thus could be old fuel?

Clearly I could use some advice before surrendering to towing the car to the shop.

[tirwin]

Well I guess you've figured out by now that 45° increments is WAY too much.

ASSUMING the following:
- No vacuum leaks
- fuel pressures in spec
- good spark
- timing set properly
- your new K-Jetronic bits are installed correctly and functioning properly

2 things: (1) if all the above is true, THEN you need to re-baseline your mixture and (2) reading your post makes me think the pressure plate is not adjusted properly.

If the above conditions aren't all true then adjusting the mixture is just trying to push a rope up a hill.

Let me see if I can find a couple of pointers. Be right back...

[tirwin]

See post from timmy2 that describes the procedure to baseline the mixture. It's post #3.

Base setting of mixture for CIS 1980SC?

Also here is the link to the general procedure for setting the mixture from Jim Williams' site:

http://jimsbasementworkshop.com/CIS/pages/test_mixtures.html

I can't put my finger on the how-to for adjusting the pressure plate. Maybe it's in the Bentley manual. Do you have one? If not, let me know.

[timmy2]

How to adjust plate...



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[RDM]

The first thing to do when you're in a hole is to stop digging. My advice would be to read Dennis' links and do a lot more research before you touch another thing under the engine cover.

Then start checking Tim's list of assumptions to make sure they're true. I would actually add one more:
Check the numbers on your CIS components to make sure they belong together.

I had a 924 WUR in my SC that no amount of adjusting would have fixed. All the mechanics that worked on the car before I got it missed the problem.

[Porsche 2]

Thanks fellows, this is all very helpful. As the engine was performing very well before connecting the transplanted harnesses and additional '80 OXS parts(Jetronic unit with relay, 15 degree temp. switch, O2 exhaust sensor, (throttle switch and frequency valve were always on the engine)), I will focus on the fuel distributor air/fuel mixture screw and bypass idle speed air adjustment screw. As I have already adjusted the air/fuel mixture screw so much, I think I should 1) "baseline" check/adjust the sensor plate stop, 2) adjust mixture screw per timmy2's 5 step description (08-24-2013). Then "baseline" the idle screw, --- any advice on how many turns to set this from closed position?

[Joe Bob]

Personally, I wouldn't be screwing with the mix w/o a gas analyzer hooked up. Throw it on the hook and take to the shop......let them set it. Bet the plugs are fouled by now or maybe the 02 sensor is kaput.

[T77911S]

oh boy, this one is going to be fun
my first thought is it sounds like you have something bad since it runs but slowly dies like it is running out of gas. (just something i am tossing around right now).
like you said, could be gas, could be fuel head problems, could be clogged gas tank. you caould also have an air leak.
with the car having sat, this also makes me lean towards something like this. CIS just does not like to sit. the plunger can get siticky, the fuel head can corrode inside, injectors can corrode, its just not good.
also, the plate adjustment is not going to make it run then die like it is doing.


first thing is to check your fuel pressures. you need to know what they are.
then make sure the frequency valve(FV) is working and check the duty cycle.
you could also check the fuel volume from the fuel pump.

i set the mixture on my 77 for 10yrs by ear. i have an LM2 and i would check after i set it and it was always good. here is how i set it.
i set the mixture so it was on the rich side of wanting to surge at idle or dip down below the idle RPM when i would blip the throttle and let it off.
set the idle to around 900, maybe 1000. rev it, if the RPM drops below 900 then comes back up, lean it out. repeat. let it idle betoween and see if it surges.


there is another option that will help you get close.
you can push up or pull down on the senor plate. if it idles better pushing up, richen the mixture.
another option to get it close. turn CW until it runs bad, turn it CCW until it runs bad (or you hear changes inthe way it runs). set it mid way or 3/4 to the rich side of the range from lean to rich.

[john walker's workshop]

did you use the '80 fuel distributor and warmup regulator or are you trying to install the '80 system on the '78 stuff? do you hear the frequency valve buzzing?

[T77911S]

if he has a FV he does not have the 78 head. i think the car is a 78, but the engine and CIS are 80. i also assume the head he is using now is the one that has been one the engine in the past.

(scares me to see JW post after me. i wonder what "I" did wrong).

[stlrj]

Originally Posted by john walker's workshop

did you use the '80 fuel distributor and warmup regulator or are you trying to install the '80 system on the '78 stuff? do you hear the frequency valve buzzing?

That's why we need more information as to how complete a transition was made of the 80 CIS system to the 78 engine. Where any of the 80 CIS components left out in the transition?

That's where we need to start.


Cheers,

Joe
82 Targa